Carbon material for electric double layer capacitor electrodes

ABSTRACT

A carbon material for electric double layer capacitor electrodes which is obtained by a heat treatment and an activation treatment of a material pitch having a softening point in a range of 150 to 350° C., a ratio of amounts by atom of hydrogen to carbon (H/C) in a range of 0.50 to 0.90 and a content of optically anisotropic components of 50% or greater. The material pitch is preferably a synthetic pitch obtained by polymerizing a condensed polycyclic hydrocarbon such as naphthalene, methylnaphthalene, anthracene, phenanthrene, acenaphthene, acenaphthylene and pyrene in the presence of an ultra-strong acid catalyst such as a hydrogen fluoride-boron trifluoride complex. Electric double layer capacitors having a high electrode density and a high electrostatic capacity per unit volume can be constructed using the carbon material.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a carbon material for electrodeswhich can be used for constructing electric double layer capacitorshaving a high electrode density and a high electrostatic capacity perunit volume.

[0003] 2. Description of the Prior Art

[0004] An electric double layer capacitor (referred to as EDLC,hereinafter) is a condenser, i.e., a device for storing electricity,utilizing a double layer formed at the interface of a solid and aliquid. The structure comprises a pair of polarizing electrodes disposedon both sides of a separator, a case for containing the electrodes, anelectrolytic liquid and a current collector.

[0005] As the material for the polarizing electrode, in general, activecarbon having a large specific surface area is used. Examples of suchactive carbon include active carbons prepared from coconut shell,petroleum pitch and polyacrylonitrile. In Japanese Patent ApplicationLaid-Open No. Heisei 9(1997)-320906, active carbons prepared from PVCand tar are described as organic materials which can be easily convertedinto graphite. In Japanese Patent Application Laid-Open No. Heisei11(1999)-293527, an electric double layer capacitor using carbon fiberderived from an optically isotropic pitch as the electrode material isdescribed.

[0006] As for the electrolytic liquid, for example, an aqueous solutionof sulfuric acid or an aqueous solution of potassium hydroxide is usedas the aqueous electrolytic liquid and an electrolytic liquid of anorganic solvent such as propylene carbonate in which an electrolyte suchas lithium perchlorate or a quaternary ammonium salt is dissolved isused as the non-aqueous electrolytic liquid.

[0007] In preparation of the active carbon, in general, an activationprocess with a gas such as water vapor or carbon dioxide is conducted.To obtain an active carbon having a large specific surface area, it isnecessary that the degree of activation be increased. However, the aboveprocess has a problem in that, since fine pores are formed by thereaction between the carbon material and water vapor or carbon dioxidewhich consumes carbon, an increase in the degree of activation resultsin a decrease in the yield and a decrease in the bulk density. Thedensity of the electrode material cannot be increased, either.

[0008] The activation can also be achieved by using an alkali metal(referred to as the alkali activation, hereinafter). An active carbonhaving a large specific surface area and a high bulk density can beobtained at a high yield in accordance with the alkali activation sincethe activation proceeds in accordance with a mechanism in which carbonis not much consumed.

[0009] Since the electrostatic capacity per unit volume of EDLC can beimproved by increasing the bulk density of the electrode material, it isconsidered that the alkali activation is the process suited forproduction of a carbon material for EDLC electrodes. However, the finestructure and the distribution of pores in the obtained active carbonare markedly different depending on the starting material and thecondition of the heat treatment and the properties of EDLC are affectedby these factors to a great degree. Therefore, it is very important thatthe starting material and the condition of the heat treatment aresuitably selected.

SUMMARY OF THE INVENTION

[0010] The present invention has an object of overcoming the aboveproblems of the conventional technology and providing a carbon materialfor EDLC electrodes which can be used for constructing EDLC having ahigh electrode density and a high electrostatic capacity per unitvolume.

[0011] As the result of studies by the present inventors on the startingmaterial, the process of treatments and the condition of treatments indetail, it was found that a carbon material for EDLC electrodes whichcan be used for constructing a high performance EDLC can be obtained byusing a specific mesophase pitch as the starting material and treatingthe pitch by a heat treatment and an activation treatment. The presentinvention has been completed based on the knowledge.

[0012] The present invention provides a carbon material for electricdouble layer capacitor electrodes which is obtained by a heat treatmentand an activation treatment of a material pitch having a softening pointin a range of 150 to 350° C., a ratio of amounts by atom of hydrogen to.carbon (H/C) in a range of 0.50 to 0.90 and a content of opticallyanisotropic components of 50% or greater, wherein the material pitch ispreferably a synthetic pitch obtained by polymerizing a condensedpolycyclic hydrocarbon such as naphthalene, methylnaphthalene,anthracene, phenanthrene, acenaphthene, acenaphthylene and pyrene in thepresence of an ultra-strong acid catalyst such as a hydrogenfluoride-boron trifluoride complex.

DETAIL DESCRIPTION OF THE INVENTION

[0013] The material pitch used in the present invention is a pitchhaving a softening point in the range of 150 to 350° C., H/C in therange of 0.50 to 0.90 and a content of optically anisotropic componentsof 50% or greater. In particular, it is preferable that the materialpitch is a synthetic pitch obtained by polymerizing a condensedpolycyclic hydrocarbon such as naphthalene, methylnaphthalene,anthracene, phenanthrene, acenaphthene, acenaphthylene and pyrene in thepresence of an ultra-strong acid catalyst such as a hydrogenfluoride-boron trifluoride complex.

[0014] In the present invention, the carbon material can be obtained bythe heat treatment and the activation treatment of the material pitchdescribed above. The heat treatment is conducted by raising thetemperature from the room temperature to a temperature in the range of400 to 800° C. in a heating oven under an atmosphere of an inert gas. Asthe inert gas, nitrogen or argon is used. When the final temperature forthe heating is lower than 400° C., the polymerization reaction of thepitch is slow and the productivity is poor. When the final temperatureexceeds 800° C., the material of the apparatus is limited and the costof production increases. Thus, temperatures outside the above range arenot preferable.

[0015] It is preferable that an infusibilizing treatment is conductedbefore the heat treatment since fusion and foaming of the pitch takeplace during the heat treatment without any treatments in advance. Forthe infusibilizing treatment, the material pitch is held in a heatingoven of the type of circulation of heated air at a temperature of thesoftening point or lower for 30 minutes to 24 hours. The fusion and thefoaming can be prevented by this treatment.

[0016] It is preferable that the granulation heat treatment methoddescribed in the following is used for the heat treatment. In a reactorkept at a temperature in the range of 400 to 800° C. under theatmosphere of an inert gas, a granular or powdery pitch obtained by aheat treatment in advance (referred to as a heat treated granular pitch,hereinafter) is placed and stirred. The material pitch is added to thepitch placed in the reactor and a heat treated pitch in a granular orpowdery form can be prepared.

[0017] In accordance with the granulation heat treatment method, theadded material pitch becomes liquid of a low viscosity by heating and isdispersed on the surface of the heat treated granular pitch placed inthe reactor in advance. The polymerization proceeds thereafter by theheat and the material pitch is finally converted into a heat treatedmaterial which does not fuse. Since the heat treated granular pitch isalways kept in the fluid condition by the stirring, gases formed by thereaction of the material pitch are quickly discharged to the outside ofthe system. Therefore, the heat treatment can be achieved efficiently ina reactor having a much smaller capacity than that used in the staticprocess without formation of foams unlike the static process in whichthe heat treatment is conducted while the material pitch is leftstanding. The polymerization proceeds while the material pitch isdispersed on the surface of the heat treated granular pitch and thematerial pitch solidifies under the shearing force formed by the fluidmovement of the heat treated granular pitch. Therefore, the productobtained after the heat treatment has an optical structure of a mosaictexture.

[0018] As the reactor used in the above process, a reactor of the tanktype equipped with a stirring apparatus which can sufficiently stir theheat treated granular pitch, a reactor of the cylinder type equippedwith a paddle for stirring or a rotary kiln can be used. When thereactor of the tank type is used, for example, a reactor equipped with astirrer having an inclined rotational axis of the stirrer bladesdescribed in Japanese Patent Application Laid-Open No. Heisei7(1995)-286181 can be used.

[0019] It is inevitable that, in the early period of the operation, aheat treat product having the structure of a flow texture produced inaccordance with the static process is used as the heat treated granularpitch placed in the reactor in advance. The heat treated product havingthe structure of a flow texture is almost completely replaced with thenewly formed heat treated product having the structure of a mosaictexture as the reaction is continued. Therefore, the heat treatedproduct having the structure of a flow texture used in the early periodof the operation little affects the active carbon obtained by theprocess.

[0020] The pitch which has been subjected to the heat treatment in theabove is then subjected to the activation treatment. The chemicalactivation using an activating agent is preferable as the activationprocess. As the activating agent used for the activation, zinc chlorideor an alkali metal compound is used. The alkali metal compounds arepreferable and potassium hydroxide, potassium carbonate and potassiumchloride are more preferable. Potassium hydroxide is most preferable.

[0021] When the temperature of the heat treatment is lower than 600° C.,the carbonation can be completed and more efficient activation can beachieved by treating the heat treated pitch by a further heat treatmentat 600° C. or higher before being mixed with the activating agent. Amore efficient activation can be achieved by pulverization of the heattreated pitch before mixing with the activating agent since the surfacearea for contact with the activating agent is increased. As thepulverizing machine, a suitable machine can be selected from thepulverizer of the impact type, the jet mill and the like machines.

[0022] The activation treatment of the pitch which has been subjected tothe heat treatment is conducted by pulverizing the product of the heattreatment into powder having diameters of 5 to 90 μm in accordance withthe above process by a pulverizer of the impact type or the like, mixingthe pulverized product with the activating agent and treating theobtained mixture by a further heat treatment.

[0023] When an alkali metal compound is used as the activating agent,the alkali metal compound in an amount in the range of 1 to 4 parts byweight per 1 part by weight of the heat treated powdery pitch isuniformly mixed with the heat treated powdery pitch and packed into areactor. For forming an electric double layer, meso pores havingdiameters of 20 to 500 Å are suited. When the ratio of the amounts byweight of the alkali metal compound to the heat treated powdery pitch issmaller than 1, pores are not formed sufficiently. When the ratio of theamounts by weight of the alkali metal compound to the heat treatedpowdery pitch exceeds 4, micro pores which cannot form the electricdouble layer and macro pores formed by expansion of the meso pores areformed in increased amounts and a problem arises in that the bulkdensity decreases. It is more preferable that the alkali metal compoundis mixed with the heat treated powdery pitch in an amount in the rangeof 1 to 3 parts by weight per 1 part by weight of the heat treatedpowdery pitch.

[0024] Then, the reactor is placed under the atmosphere of an inert gassuch as nitrogen. The temperature of the reactor placed in a heatingoven is raised from the room temperature to a temperature in the rangeof 400 to 900° C. and kept at this temperature for 1 to 20 hours. Whenthe reaction temperature is lower than 400° C., the reaction does notproceed sufficiently and a sufficient degree of activation is notachieved. When the reaction temperature exceeds 900° C., a problemarises in that the alkali metal component such as the metallic potassiumis separated and scattered and the reaction apparatus is corroded. It ismore preferable that the activation treatment is conducted at atemperature in the range of 600 to 800° C.

[0025] Then, the reaction mixture is cooled to the room temperature,placed into an alcohol such as 2-propanol to remove the alkali metalcomponent, filtered, repeatedly washed with water until the filtratebecomes neutral and dried. The active carbon which is used as the carbonmaterial for electric double layer capacitor electrodes of the presentinvention can thus be obtained.

[0026] As the properties required for EDLC, in particular, theelectrostatic capacity per unit volume is important. Adsorption of theelectric double layer, i.e., adsorption of the electrolytic liquid tothe inner surface of the pores of the active carbon, is related to theelectrostatic capacity and pores suited for diffusion and adsorption ofthe substance to be adsorbed are necessary. Therefore, the electrostaticcapacity per unit volume can be improved by using the active carbonhaving a greater fraction of such pores.

[0027] In the active carbon obtained in accordance with the process ofthe present invention, pores suited for the electric double layer areefficiently formed and the above requirements for the properties aresufficiently satisfied. Specifically, an anisotropic carbon having ahigh real density is obtained by the heat treatment of the materialpitch described in the present invention. An active carbon in which manysuitable pores are formed can be efficiently produced by furtheractivating the above carbon. EDLC in which the above active carbon isused for the electrode material exhibits an electrostatic capacity perunit weight which is the same as or greater than that of active carbonsobtained in accordance with conventional processes. Since the bulkdensity of the obtained active carbon is high, an excellent electrodematerial having an electrostatic capacity per unit volume much greaterthan that of materials obtained in accordance with conventionalprocesses, i.e., 30 F/cc or greater, can be obtained.

[0028] As will be shown in examples in the following, in accordance withthe present invention, a carbon material for EDLC electrodes which canbe used for constructing EDLC having a high electrode density and a highelectrostatic capacity per unit volume can be obtained by the heattreatment and the activation treatment of a pitch having a softeningpoint in the range of 150 to 350° C., a ratio of amounts by atom ofhydrogen to carbon (H/C) in the range of 0.50 to 0.90 and a content ofoptically anisotropic components of 50% or greater, in particular, asynthetic pitch obtained by polymerizing a condensed polycyclichydrocarbon such as naphthalene, methylnaphthalene, anthracene,phenanthrene, acenaphthene, acenaphthylene and pyrene in the presence ofan ultra-strong acid catalyst such as a hydrogen fluoride-borontrifluoride complex.

EXAMPLES

[0029] The present invention will be described more specifically withreference to examples in the following. However, the present inventionis not limited to the examples.

Example 1

[0030] A mesophase pitch (the softening point: 235° C.; H/C=0.65; thecontent of optically anisotropic components: 100%; the yield ofcarbonation: 87%) was synthesized by polymerization of naphthalene inthe presence of the hydrogen fluoride-boron fluoride complex. For theheat treatment of the pitch, the temperature was raised to 530° C. at arate of 5° C./minute under the atmosphere of nitrogen and kept at 530°C. for 1 hour. Then, after the temperature was lowered to the roomtemperature, the product was pulverized and a heat treated mesophasepitch having an average particle diameter of about 0.5 mm was obtained.

[0031] The obtained pitch was pulverized by a pulverizer of the impacttype and a powder having an average diameter of 15 μm was obtained. Theobtained powder was heated in a tubular oven under the atmosphere ofnitrogen while the temperature was raised to 700° C. at a rate of 5°C./minute and kept at 700° C. for 4 hours. After the temperature waslowered to the room temperature, potassium hydroxide in an amount of 2parts by weight per 1 part by weight of the heat treated powder wasadded to the heat treated powder and uniformly mixed together. Theresultant mixture was heated under the atmosphere of nitrogen while thetemperature was raised to 700° C. at a rate of 5° C./minute and kept at700° C. for 2 hours. After the temperature was lowered to the roomtemperature, the product was placed in 2-propanol. Filtration andwashing with water were repeated until the filtrate became neutral andan active carbon was obtained.

[0032] Electrodes were prepared using the obtained active carbon inaccordance with the following process and evaluated.

[0033] Electrodes were prepared by mixing the active carbon, anelectrically conductive filler (ketjen black) and a binder (Teflon) inamounts such that the ratio of the amounts by weight of the activecarbon, the filler and the binder was 90:10:5. A cell of the twoelectrode type made of glass was used. A separator made of glass fiberis placed between a pair of the electrodes and placed into the cell. Asthe electrolytic liquid, propylene carbonate in which tetraethylammoniumtetrafluoroborate ((C₂H₅)₄NBF₄) was dissolved in a concentration of 0.65mole/liter was used.

[0034] Under the atmosphere of argon and at the room temperature, thecell was charged with a constant current of 5 mA/g until the voltagefinally reached 3 volts. The charged cell was then discharged with aconstant current of 5 mA/g until the voltage became zero volt. Theelectrostatic capacity per unit volume was calculated by multiplying theelectrostatic capacity per unit weight C by the density of theelectrode. The electrostatic capacity per unit weight C is expressed asC=IΔT/ΔV, wherein I represents the average value of the current per unitweight of the electrode during the discharge, ΔT represents the time oflowering the voltage and ΔT represents the lowered range of the voltage.As the result, the electrostatic capacity was found to be as excellentas 32.4 F/cc.

Example 2

[0035] A mesophase pitch was synthesized in accordance with the sameprocedures as those conducted in Example 1.

[0036] For the heat treatment of the pitch, the temperature was raisedto 530° C. at a rate of 5° C./minute under the atmosphere of nitrogenand kept at 530° C. for 1 hour. Then, after the temperature was loweredto the room temperature, the product was pulverized and a heat treatedmesophase pitch having an average particle diameter of about 0.5 mm wasobtained.

[0037] Into a reactor of the tank type having a diameter of 170 mm and aheight of 170 mm and equipped with a stirrer, 200 g of the mesophasepitch obtained above was placed in advance as the heat treated granularpitch. The temperature was raised to 550° C. under stirring under thestream of nitrogen and the mesophase pitch obtained above was added tothe reactor at a rate of 10 g per minute until the total amount reached300 g.

[0038] After the addition was completed and the temperature was kept at550° C. for further 10 minutes, the reactor was cooled and the contentof the reactor was taken out. A granular heat treated product wasobtained in an amount of 400 g. The obtained heat treated product wasused as the heat treated granular pitch and the same heat treatment asthat described above was conducted. This treatment was repeated 7 timesand a heat treated mesophase pitch in which about 99% of the amount hadbeen replaced with the desired heat treated granular pitch was obtained.The obtained pitch was pulverized by a pulverizer of the impact type andpowder having an average diameter of 15 μm was obtained. The obtainedpowder was heated in a tubular oven under the atmosphere of nitrogenwhile the temperature was raised to 700° C. at a rate of 5° C./minuteand kept at 700° C. for 4 hours.

[0039] The activation treatment and the evaluation of the electrodeswere conducted in accordance with the same procedures as those conductedin Example 1. The electrostatic capacity was obtained in accordance withthe same procedures as those conducted in Example 1 and found to be 44.3F/cc.

Example 3

[0040] A mesophase pitch was synthesized in accordance with the sameprocedures as those conducted in Example 1 and pulverized by apulverizer into a powder having an average diameter of 20 μm. Theobtained powder of mesophase pitch was subjected to the infusibilizingtreatment and the heat treatment.

[0041] For the infusibilizing treatment, a heating oven of the type ofcirculation of dry air was used. Into a vat made of stainless steel, 20g of the powder of mesophase pitch was placed in a manner such that theentire bottom face of the vat was covered with the mesophase pitch. Thevat was then kept in a reaction oven at a temperature of 200° C. for 10hours. The obtained product which had been subjected to theinfusibilizing treatment was subjected to the activation treatment inaccordance with the same procedures as those conducted in Example 1 andthe obtained electrode was evaluated. The electrostatic capacity wasobtained in accordance with the same procedures as those conducted inExample 1 and found to be 30.2 F/cc.

Comparative Example 1

[0042] A powder of carbon of coconut shell was subjected to theactivation treatment while the temperature was raised to 900° C. at arate of 5° C./minute under the atmosphere of nitrogen and kept at 900°C. for 3 hours under a stream of nitrogen containing 10% of water vapor.Using the obtained product as the electrode material, the electrostaticcapacity was obtained in accordance with the same procedures as thoseconducted in Example 1 and found to be 16.4 F/cc.

Comparative Example 2

[0043] A powder of carbon of coconut shell and potassium hydroxide in anamount of 2 parts by weight per 1 part by weight of the carbon wereuniformly mixed together. The resultant mixture was subjected to theactivation treatment while the temperature was raised to 700° C. at arate of 5° C./minute under the atmosphere of nitrogen and kept at 700° Cfor 2 hours. Using the obtained product as the electrode material, theelectrostatic capacity was obtained in accordance with the sameprocedures as those conducted in Example 1 and found to be 12.4 F/cc.

Comparative Example 3

[0044] Ethylene tar and furfural each in an amount of 1 part by weightwere mixed together and 5% of p-toluenesulfonic acid was added to theresultant mixture. A powder of carbon obtained by the reaction of themixture at 150° C. was subjected to the activation treatment while thetemperature was raised to 900° C. at a rate of 5° C./minute under theatmosphere of nitrogen and then kept at 900° C. for 3 hours under astream of nitrogen containing 10% of water vapor. Using the obtainedproduct as the electrode material, the electrostatic capacity wasobtained in accordance with the same procedures as those conducted inExample 1 and found to be 18.5 F/cc.

Comparative Example 4

[0045] Ethylene tar and furfural each in an amount of 1 part by weightwere mixed together and 5% of p-toluenesulfonic acid was added to theresultant mixture. A powder of carbon obtained by the reaction of theabove mixture at 150° C. and potassium hydroxide in an amount of 2 partsby weight per 1 part by weight of the carbon were uniformly mixedtogether. The resultant mixture was subjected to the activationtreatment while the temperature was raised to 700° C. at a rate of 5°C./minute under the atmosphere of nitrogen and then kept at 700° C. for2 hours. Using the obtained product as the electrode material, theelectrostatic capacity was obtained in accordance with the sameprocedures as those conducted in Example 1 and found to be 5.5 F/cc.

Comparative Example 5

[0046] A coal tar pitch having a softening point of 76° C. and H/C of0.55 was placed into an autoclave. The air in an amount of 2 liters per100 g of the tar was blown into the autoclave at 340° C. The reactionwas allowed to proceed for 1 hour and a 100% optically isotropicmodified pitch having a softening point of 243° C. was obtained. H/C inthe modified pitch was 0.48.

[0047] The modified pitch was pulverized and a powder having diametersof 200 μm or smaller was obtained. The powder in an amount of 10 g wasplaced in a porcelain dish and the dish was placed in a muffle oven. Thetemperature was raised from 150° C. to 320° C. at a rate of 0.5°C./minute under a stream of air of 1 liter per minute and kept at 320°C. for 10 minutes and then the product was taken out. In accordance withthe same procedures as those conducted in Example 3, the obtainedproduct which had been subjected to the infusibilizing treatment wasplaced into a tubular oven and heated while the temperature was raisedto 700° C. at a rate of 5° C./minute and kept at 700° C. for 4 hours.The obtained powder which had been subjected to the heat treatment andthe infusibilizing treatment was subjected to the activation treatmentin accordance with the same procedures as those conducted in Example 1and the obtained electrode was evaluated. The electrostatic capacity wasobtained in accordance with the same procedures as those conducted inExample 1 and found to be 19.5 F/cc.

[0048] The results of Examples 1 to 3 and Comparative Examples 1 to 5are summarized in Table 1. TABLE 1 Electrostatic Method capacity per ofunit volume Starting material activation (F/cc) Example 1 mesophasepitch KOH 32.4 Example 2 mesophase pitch KOH 44.3 Example 3 mesophasepitch KOH 30.2 Comparative coconut shell water vapor 16.4 Example 1Comparative coconut shell KOH 12.4 Example 2 Comparative ethylene tarpitch water vapor 18.5 Example 3 Comparative ethylene tar pitch KOH  5.5Example 4 Comparative isotropic pitch KOH 19.5 Example 5

What is claimed is:
 1. A carbon material for electric double layercapacitor electrodes which is obtained by a heat treatment and anactivation treatment of a material pitch having a softening point in arange of 150 to 350° C., a ratio of amounts by atom of hydrogen tocarbon (H/C) in a range of 0.50 to 0.90 and a content of opticallyanisotropic components of 50% or greater.
 2. A carbon material forelectric double layer capacitor electrodes according to claim 1, whereinthe material pitch is obtained by polymerizing a condensed polycyclichydrocarbon or a substance containing the condensed polycyclichydrocarbon in a presence of a hydrogen fluoride-boron trifluoridecomplex.
 3. A carbon material for electric double layer capacitorelectrodes according to claim 1, wherein the heat treatment comprisesheating the material pitch at 400 to 800° C. under an atmosphere of aninert gas.
 4. A carbon material for electric double layer capacitorelectrodes according to claim 1, wherein the heat treatment comprisesplacing a granular or powdery pitch obtained by a heat treatment into areactor heated at 400 to 800° C., adding the material pitch to the pitchplaced in the reactor under stirring and conducting the heat treatment.5. A carbon material for electric double layer capacitor electrodesaccording to claim 1, wherein the activation treatment comprises mixingthe material pitch which has been subjected to the heat treatment withan alkali metal compound in an amount of 1 to 4 parts by weight per 1part by weight of the material pitch and heating an obtained mixture at400 to 900° C. under a stream of an inert gas.